54 
AN INVESTIGATION INTO THE 
to assume a more lath-shaped development than in the case of the granites. 
The laths running as they do in all directions through the rock, probably have a 
tendency to bind the rock more firmly together than when the feldspar has 
a more equi-dimensional development, as in the granites. The rock has a 
hypidiomorphic structure, and, like the granites described in this paper, is 
perfectly massive. 
A color-process photograph of a polished surface of this rock is shown in 
Plate XIII A, and a photomicrograph of a thin section taken between crossed 
nicols in polarized light and magnified 30 diameters is to be seen in Plate XIII B . 
Three square prisms of the rock were used, and five determinations of verti- 
cal compression with three of lateral extension were made. The results are 
given in the table on page 53. 
The averages of the results obtained are as follows : 
E= 9,746,00; ff= 0.2583; D= 6,750,000; C= 3,872,600. 
The results obtained for the three measurements on prism a were practically 
identical. The figures obtained for the compressibility of c are little higher 
and those for b are considerably lower. The difference between the highest 
and the lowest values obtained for D amounts to 1,110,000 pounds, but the 
difference, if the results of the single measurement on 6 be omitted from 
consideration, amounts to only 330,000 pounds. 
The stress- strain curves plotted from the measurements obtained from the 
prism a are given in figure 21, and show that the elasticity of the rock is 
of a very high order. In this figure I represents vertical compression and 
II lateral extension. 
900O 
IOOO 
28O 
FIG. 21. Mount Johnson Essexite. Stress-strain curves 
